In DCR's, a received signal from the antenna passes through radio frequency (RF) filtering and then through an RF amplifier. The filtered and amplified signal is subjected to a quadrature mix to baseband I and Q quadrature signals. Typically, the I and Q signals are low pass filtered, amplified, and digitized by an analog-to-digital converter (ADC). The I and Q signals are processed by digital signal processing hardware and software and then passed to a digital-to-analog converter (DAC) to reconstruct usable demodulated signals or to a computer for further processing. The “tuned frequency” of a DCR is the frequency of the local oscillator from the frequency synthesizer.
The I and Q signals in a DCR only represent the received signals to the extent that they are in perfect quadrature, i.e., to the extent that they are of equal amplitude and separated in phase by exactly 90 degrees. If perfect quadrature does not exist, distortion products result when attempts are made to extract useful information from the I and Q signals. Techniques for determining and correcting these quadrature errors are known, but typically only over a narrow passband. For wideband receivers, some of these techniques cannot correct for differential errors in gain and phase across the receiver passband.
Quadrature errors are frequently a function of how perfectly the baseband low pass filters for the I and Q signals match each other. Any mismatch between the filters produces distortion products in demodulated outputs. The distortion products are typically well within what can be tolerated in a single signal receiver. However, these filter mismatches have far more serious consequences in a wideband receiver where multiple narrowband signals are captured. Receivers of this type are employed, for example, in cellular telephone base stations and by government agencies tasked with monitoring the usage of the RF spectrum. The advantage of this architecture is that a single “hardware” receiver can intercept multiple signals and they can then be individually extracted with a “software” receiver residing in a computer. Current receivers of this type are implemented with a superheterodyne architecture. A need exists for methods of reducing filter mismatches, thereby improving wideband DCR performance.